In a conventional mechanical screen printing machine driven by a central motor, the rotary motion of the central motor drives a feeder, an infeed table feeder, a side alignment device, a print cylinder, a screen carrier, and a sheet delivery device of the mechanical screen printing machine to operate.
In the above-structured mechanical screen printing machine, a direct-driven center shaft and flying gears provided on left and right sides of the print cylinder's axis together form a driving means. And, through engagement of the flying gears with two sprocket bars, the screen carrier is driven on both sides to move forward for printing or move backward for flooding.
However, the first conventional mechanical screen printing machine is subject to interference of the mechanical accuracy of synchronous drive of the screen carrier and the print cylinder. This is because the mechanical driving is realized via two tooth contact points between the screen carrier and the gears on the left and the right side of the print cylinder axis, and the mesh between gears will change with different rotational speeds. Therefore, accuracy in synchronization on each point of movement is required to guarantee the position between the printing material sheet, which is fixed to the print cylinder surface and rotating along with the print cylinder, and the screen carrier that horizontally moves above the print cylinder. Also, the gears having operated for a long period of time will wear down to result in increased backlash. And, inaccuracy in synchronization will cause printing inaccuracy when the backlash is large
Another conventional mechanical screen printing machine using the unique stop-cylinder principle has been developed to avoid printing inaccuracy by driving the print cylinder via a screen carrier on both sides of the print cylinder, such as tooth racks. With this arrangement, there is only one contact point on each of the two tooth racks and thus the screen carrier and the print cylinder are accurately synchronized with one another without being influenced by printing speed and mass force.
While the screen printing machine using the stop-cylinder principle realizes the high synchronization accuracy, it is still difficult to control the deviation between the screen movement and the print cylinder movement because the screen carrier has a much shorter run in relation to the 360-degree cylinder movement. Thus, it is not possible at all for the screen carrier movement to be independent of the print cylinder motion on the mechanical screen printing machine.
To overcome the disadvantages of the previous two conventional mechanical screen printing machines, another screen printing machine having separate servo drives is developed. Please refer to FIGS. 1 and 2, the conventional screen printing machine with separate servo drives is driven to operate by independent drives that are electronically synchronized and controlled. More specifically, there is a first driving source 10 for driving a feeder 101, an infeed table 102 and a side alignment device 103; a second driving source 11 for driving a print cylinder 111 and a rotary shaft 112; a third driving source 12 for driving a screen carrier 121; and a fourth driving source 13 for driving a delivery belt 131. Wherein, the third driving source 12 drives free-wheel gears 124 provided on left and right sides of the screen carrier 121 via gears 123 connected to two sides of a transfer shaft 122.
However, in the above-structured conventional servo-driven screen printing machine, the screen carrier 121 is still driven via meshing of gears 123 with gears 124, and the problem of tooth backlashes still exist. Therefore, with the known servo drive designs, it is still not possible to achieve an accuracy of +/−0.03 mm covering the entire speed range.
Further, all the above-mentioned conventional screen printing machines involve a complicated structure that includes multiple sets of gears, tooth racks, cams and chains, which not only results in increased part assembling labor and time, but also accumulated part tolerance that prevents the machine from meeting the requirement for synchronization and printing accuracy.